We are investigating organic porous polymer monoliths (PPMs)
that are used in capillary electrochromatography
(CEC) applications. More specifically, we are investigating fundamental
properties of lauryl acrylate polymers to better understand the nature of the analyte, stationary phase, and mobile phase interactions.
This is approached by measuring the diffusion coefficients of a variety of analytes with different retention factors, under different
conditions, to determine the relationship between diffusion and retention as
well as to examine the thermodynamics of the retention and diffusion processes.
In traditional chromatography systems, utilizing familiar stationary phases
comprised of C18 bonded to silica surfaces, the relationship between
diffusion and retention has typically been described with the Knox equation
given below in eq 1 where Deff is the effective diffusion coefficient of the analyte in the presence of the stationary phase, gm is the column obstruction factor, Dm is
the diffusion coefficient of the analyte in the
mobile phase, k is the retention factor, and gsDsis the
kinetic parameter, comprised of the obstruction within the stationary phase and
diffusion within the stationary phase.

Deff = (gmDm+ kgsDs)/(1+k) (1)
(1+k) Deff /Dm = gm + kgsDs/Dm (2)

Eq 2 is simply a rearrangement of eq 1 and shows that a plot of the left hand side versus k
should result in a straight line where the intercept will yield gm and the slope will yield the kinetic parameter.
However, all investigations using C18 bonded to silica supports have
resulted in plots that show negative deviations from linearity. There are two
explanations for this behavior. One is that the kinetic parameter is not a
constant but rather a function of retention. The second is that the Knox theory
is in fact incorrect and does not account for the two types of diffusion that
occur within the stationary phase. Our results for a lauryl acrylate PPM stationary
phase, utilizing a 75:25 ACN:5 mMTris mobile phase, yields dramatically different
results. Ideal Knox behavior is observed. A straight line is achieved, and the
kinetic parameter is greatly reduced from those typically seen in silica based
stationary phase studies. We have attributed these differences to two
properties of the PPM that set it apart from silica based stationary phases.
The adherence to linearity is attributed to a relative lack of mesopores in the polymer monolith which
in turn results in far less stagnant mobile phase zones. Furthermore the greatly
reduced kinetic parameter is attributed to the structure of the polymer surface
which is markedly different from the surface of a C18 bonded phase
in terms of the likely presentation of the carbon chains but also in terms of
the distribution of charged moieties. Our results, obtained for a set of 25 analytes covering a retention range of less than one to
over 12 is shown in Fig. 1. These results have just been submitted for peer reviewed publication.

In another facet of our
study, we are examining the thermodynamics of retention on the same phase. Analyte retention is measured as a function of temperature,
covering a temperature range of 25-60 degrees. From the plots of ln k versus the inverse of the temperature, van't Hoff plots, the enthalpy and entropy of retention can
be obtained. We are currently conducting experiments to determine the phase
ratio of these polymer preparations. When completed, we will be able to calculate
the entropy of retention. We expect to complete this data analysis very soon
and a manuscript draft is in preparation.

This grant has supported
students in the summers of 2011 and 2012. To date nine individual students have
been involved with this research. A total of five students have worked on this
project in each summer and three of these were fully supported by this grant.
The nature of the project requires a substantial amount of careful, time consuming data collection which in turn requires a
sizeable research team. We would not now be preparing to publish two distinct
papers as a result of this research if fewer students had participated. Two
students worked on this research in both the summers of 2011 and 2012 and the
intervening academic year. One of these is currently applying to graduate
schools to complete PhD studies in analytical chemistry while spending his
senior year abroad. All students who participated in this research in the
summer of 2011 continued for all or part of the 2011-2012 academic year. Of the five students who participated in the summer of
2012, three are currently still participating. Of those who participated in the
summer of 2011, two presented posters at the 243rd American Chemical
Society meeting in March, held in San Diego. One of the others gave a talk at Pittcon 2012 in March in Orlando, and the other two copresented a poster at Pittcon
2012. Of the five who participated in the summer of 2012, two have oral talk
abstracts submitted for Pittcon 2013 and two have
poster abstracts submitted. Two postdoctoral associates, funded by other
sources, have also participated in research related to this grant. They have
also been partially responsible for supervising these undergraduate
researchers. The first postdoc has recently completed one year in this
laboratory and has now begun a tenure track appointment at a predominantly
undergraduate institution. The second postdoc is at the beginning of her
appointment in this laboratory. Funding from the ACS PRF has revitalized my
undergraduate research program.